Hello friends!,

I’m dealing with a highly multireference diatomic electronic state (because that’s what I do) and having some CCSD convergence issues. The ^4Pi state of FeH seems to behave with some basis sets and not with others. Using cc-pwCVQZ, I get a good geometry and correlation energy/relative energy to other electronic states that matches what I have observed in molpro. At cc-pwCVTZ and cc-pwCV5Z the CCSD energy is 0.2 Hartrees to high. I have looked at the MO coefficients of the ROHF wave function - it looks like the correct state and ROHF energy, and a stability analysis doesn’t find a better set of orbitals. I guess the problem is with the CC convergence. I tried turning off DIIS and ccenergy went kray.

Is it possible in PSI4 to delay when DIIS works, and/or play with the number of expansion vectors? This does a great job of solving fussy CC convergence problems in molpro and cfour.

Thanks!

Nate

PS: input file

molecule {

0 4

Fe

H 1 1.562441

}

set {

reference rohf

docc [7, 0, 3, 2]

socc [1, 1, 0, 1]

scf_type pk

print_mos true

cachelevel=0

maxiter=500

rms_force_g_convergence 2

freeze_core false

}

basis{

assign Fe cc-pwcvtz

assign H cc-pwcvtz

}

set scf df_scf_guess false

set scf guess gwh

set ccenergy r_convergence 10

optimize(‘ccsd’)